This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
The ultimate goal of mobile broadband should be ubiquitous and sustainable provision of non-limiting data rates to everyone and everything at every time. To achieve this goal, Ultra-Dense Network (UDN) has been proposed as an important next step following the successful introduction of Long Term Evolution-Advanced (LTE-A) for wide-area and local-area access.
Through sufficient provision of ANs and operation at very wide bandwidths in the millimeter-wave bands, UDN creates ubiquitous access opportunities which—even under realistic assumption on user density and traffic—provide users with the desired data rates.
In a UDN, sufficient provision is achieved by an extremely dense grid of ANs. To be specific, inter-AN distances may be in the order of tens of meters or below. In indoor deployments, one or even multiple ANs may be arranged in each room.
As a prerequisite for communications via a UDN, backhaul link synchronization between ANs in the UDN shall be achieved not only in time domain but also in frequency domain. The time domain synchronization prevents Uplink/Downlink collision (assuming TDD duplex is applied for the UDN) and enables intelligent inter-cell interference coordination. The frequency domain synchronization enables low-complexity frequency error estimation and accordingly reduces handover latency.
Due to the fact that there is typically no wired connection between ANs in a UDN, wired backhaul based synchronization techniques (including, for example, packet based synchronization scheme, Global Navigation Satellite System synchronization scheme, etc.) commonly used in traditional cellular networks are no longer applicable to UDNs. Instead, wireless backhaul based synchronization techniques have been proposed particularly for UDNs.
Generally, the existing wireless backhaul based synchronization techniques can be classified into two types: hierarchical synchronization (see Reference 1) and distributed synchronization (see Reference 2).
According to an example of hierarchical synchronization as illustrated in FIG. 1, one of multiple ANs in a UDN is selected as a synchronization root and assigned a synchronization level 0. Two other ANs adjacent to the level-0 AN are each assigned a synchronization level 1 and synchronized to the level-0 AN directly by referring to a synchronization reference conveyed by a synchronization signal broadcast by the level-0 AN. Likewise, ANs adjacent to the level-1 ANs are each assigned a synchronization level 2 and synchronized to the level-1 ANs directly by referring to the synchronization reference conveyed by synchronization signals broadcast by the level-1 ANs, and so on.
In this manner, a synchronization tree is formed from the level-0 AN (i.e., the root AN) to level-n (n>0) ANs (i.e., leaf ANs). The level-1 ANs can be synchronized to the level-0 AN directly by referring to the synchronization reference originating from the level-0 AN directly, while higher-level ANs can be synchronized to the level-0 AN indirectly by referring to the synchronization reference originating from the level-0 AN indirectly.
Typically, an aggregation node (AGN) which has access to an accurate synchronization source (e.g., a Global Position System (GPS) source) may be selected as the root AN. Practical application of the hierarchical synchronization scheme can be found in device-to-device communications in out-of-coverage scenario (see Reference 3).
According to an example of distributed synchronization as illustrated in FIG. 2, each of ANs on the one hand broadcasts a synchronization signal and on the other hand receives synchronization signals broadcast by its neighboring ANs. Based on its received synchronization signals, the AN can update its own synchronization, for example, by performing an arithmetic averaging operation on synchronization references carried by the received synchronization signals. During the initial setup stage of distributed synchronization, iterations are thus required to reach a convergence.
Due to the important role of the root AN in a UDN employing the hierarchical synchronization scheme, synchronization can no longer be maintained in the UDN if the root AN (i.e., the level-0 AN) is down. At the first glance, it seems a feasible solution to such a crisis that a level-1 AN in the UDN autonomously takes the place of the failed level-0 AN. This solution however may cause synchronization collisions in the UDN in various forms.
By way of example, FIG. 3 depicts a scenario where a direct synchronization collision occurs between level-1 ANs. As illustrated, two level-1 ANs exist in the UDN and are located in the proximity of each other (i.e., they can receive synchronization signals from each other). In case the level-0 AN is down, these two level-1 ANs autonomously act as independent new root ANs. Due to the intrinsic difference between their local synchronization references in time/frequency domain, the new root ANs themselves are not synchronous.
In another scenario shown in FIG. 4, two level-1 ANs are not located in the proximity of each other. Accordingly, in case the level-0 AN is down and the level-1 ANs autonomously act as independent new root ANs, they are unaware of the synchronization difference between each other. However, a level-2 AN, which can receive synchronization signals from both the level-1 ANs, may be confused with inconsistent synchronization signals broadcast by the two level-1 ANs. Thus, an indirect synchronization collision occurs between level-1 ANs.